Reducing level of interference chemicals in water circulation of wood-based fibre suspension processes

ABSTRACT

The invention relates to a method for reducing the amount of interference substances in the water circulations of processes involving wood-based fibre suspensions by binding the disturbance substances to the fibres by means of cationic starch. The amount of the said starch added to the fibre suspension is approximately 0.04-0.5% of the fibre weight.

Several changes have taken place during the past few years in the watercirculations of processes dealing with wood-based fibre suspensions,especially in the case of water circulations connected to papermaking:

Environmental and social pressures call for reductions in effluents tothe water system. This leads to shortened water circulations and theirpartial closing off.

The need to save expensive fibre raw material calls for increasing useof recycled fibre. The use of recycled fibre has increased significantlyand there are countries where the legislation makes it compulsory to usesuch material. For the same reason the use of groundwood and othermechanical pulps is on the rise.

As a means of cutting costs, increasingly higher proportions of the moreexpensive filler and coater pigments in the manufacturing of paper andboard are being replaced by CaCO₃. This requires a shift over fromacidic processes to neutral processes.

Due to all the above reasons, increasing amounts of what may be calledinterference chemicals are becoming concentrated in the process watercirculations. The interference materials are chemically a highlyheterogenous group of compounds. Typical examples of these are woodextracts, neutral or anionically charged, substances in dissolved orcolloidal form arising during cellulose production or originating fromthe wood.

In addition, coating binder residues, synthetic polymers, anionicemulgators, etc., end up in the water circulation of the paper machinein the course of manufacturing of coated papers.

De-inked recycled material can never equal virgin fibre raw material interms of purity as it always contains de-inking chemical and inkresidues.

The aforementioned interference substances impair i.a. the runnabilityof the paper machine by increasing the frequency of web breaks, byblocking up the wires, by contaminating the press rolls and by reducingpaper quality in terms of increasing number of holes. The interferencesubstances reduce retention by reducing the efficiency of the availableretention system as impurities bind to the retention substance andthereby bind part of the functional groups of the said retentionsubstance.

Broadly defined, all chemicals used in the above processes becomeinterference substances once they become concentrated in the watercirculation. In its more emphasised form, this problem occurs expresslyin paper machines.

Traditionally, paper manufacturers have used alum to combat the effectof interference substances. Alum efficiently binds interferencesubstances to the fibres and thereby inactivates them. The effect ofalum is based on the fact that within the pH range of 4.2-5.0 alum formsan abundance of positively charged cations which bind and neutralisemainly negatively charged interference substances. In neutral papermanufacturing processes alum is ineffective and this is why othersolutions must be resorted to.

Another method is to use talc. Talc binds impurities to its surface andin favourable conditions the interference substances leave the systemalong with the paper web. However, talc does not work very well if theconcentration of impurities is high.

Dispersing agents are used with the purpose of dispersing theinterference substances into the circulation water. A shortening of thewater circulations leads to situations in which the concentrations ofthe interference substances rise to high levels in the circulationwater, which in turn reduces retention and on the other hand makes itpossible for fresh accumulations to occur, thereby reducing productquality.

As alum is a cationic product, there is a trend in neutral processpapermaking to replace it with short-chained cationic polymers; the bestknown among these are the poly-DADMAC products (i.e.poly-diallyl-dimethylammoniumchloride). These polymers typically possessa high cationic charge (meqv/g) and their molecular weight is relativelylow when compared to synthetic products traditionally used as retentionpolymers. Corresponding other cationic binding chemicals include variouspolyamine polymers, etc. It is believed that the high cationicshort-chained polymers function by binding the interference substancesto the fibre material and consequently the interference substances leavethe system continuously along with the fibres. Products of this type areadded to the system at an early enough stage to prevent binder substanceparticles, for instance, from forming harmful agglomerations (whitepitch).

There are several ways to measure interference substances in thecirculating waters of paper machines and the like but all are more orless indirect. One of these methods is based on determining the chargestate (the interference substances are generally negatively charged);i.e. the higher the negative charge, the higher the concentration ofinterference substances. When added to the system, the highly cationic,short-chained polymers reduce the negative charge (=the amount ofnegatively charged colloids of interference substances). This change incharge state also depicts the change in the concentration ofinterference substances.

In laboratory conditions, the efficiency of cationic polymers can bemeasured by determining the residual turbidity of the fibre material(mechanical stock, reject stock, etc.) filtered through the wire orfilter paper before and after chemical treatment. It is also possible tomeasure the chemical oxygen demand (COD) or the conductivity of thesystem.

The efficient dosage of the said substances depends very much on thesystem; the dirtier the system, the higher the required dosage.Generally, dosages amounting to 0.05-0.4% of the dry weight of the fibregives good results. In the case of paper machines, the point of dosageis typically immediately after the stock storage tank before the otherchemicals are added to the system.

Starches are used in papermaking as paper strength improving component.Cationised starch in the form of stock starch or cationic surface gluebinds better to the fibres and does not end up in the circulating watersof the paper machine. Thus, it reduces the COD-load of waste andcirculating waters.

Surprisingly it has been observed that when the charge state of starchis raised by cationising starch ethers with2,3-epoxy-propyl-trimethylammoniumchloride to the level of 2-3 meqv ofN/g, the said highly cationised starches function in the manner of alumor a pDADMAC chemical by binding anionic colloidal substances present inpapermaking regardless of whether the said substances originate from amechanical stock or coated reject. Contrary to alum, the starches inaccordance with the invention function regardless of the pH level andcan thus also be used in neutral papermaking processes.

The invention is described in the light of the following comparativeexamples.

The examples used in the comparison are polyDADMAC (Basf: Fixiermittel,Nalco: 8674, Mobil: Certrex 340), polyimine (Basf: Polymin SK), andpolyaluminiumchloride (Oulu Oy: Oulupac 180).

In all the laboratory tests conducted, the pulp was stirred for 2minutes following the proportioning of chemicals therein after which itwas filtered through a 150 μm wire screen. The filtrates were measuredfor their turbidity, charge state and/or chemical oxygen demand (COD).

Retention tests were conducted using a Dynamic Drainage Jar tester(TAPPI, February, 1976, Vol. 59, No. 2, pp. 67-70). The retention testsconsisted of first adding the interference removing agent to the stockand allowing it to mix for 1 min 30 secs, then of adding the stockstarch and allowing this to mix for 30 secs, and finally of adding theretention substance (Allied Colloids: Percol 47). Filtering wascommenced 30 secs after the addition of the retention substance. Thefiltrates were measured for their COD values.

Turbidity was measured in TE/F units (Dr. Lange LTP 5) and the chargestate was determined by titrating using a PCD-02 charge state detector(Amutek) with an 0,001N MGC (methyl glycol kitosane) solution as thetitration solution. COD was determined using a test kit (Dr. Lange).

All dosages were determined as active substance per dry matter of stock.

EXAMPLE 1

Stock: coated reject, pH 4.6

Turbidity, charge state, COD.

Two batches of cationic starch differing in degree of purity werecompared to commercial polyDADMAC. The cationisation degree of both was2.7 meqv/g.

    ______________________________________                                                 Dosage                                                                              Turbidity Charge state                                                                             COD                                                %     (1:10)    μeqv/l  mg/l                                      ______________________________________                                        0                  45        1921     658                                     PolyDADMAC 0.1     18.8      1400     585                                     Basf       0.3     14.9      613      540                                                0.6     13.4      160      483                                     Cationised 0.1     18.4      1579     653                                     starch     0.3     12.9      871      603                                     (non-purif.)                                                                             0.6     15.6      454      618                                     Cationised 0.1     17.9      1506     618                                     starch     0.3     11.9      776      570                                     (purified) 0.6     13.8      291      525                                     ______________________________________                                    

EXAMPLE 2

Stock: Groundwood pulp, pH 7

Retention test

Points 1-3 served as comparison points for the test points 4-6 whichinclude cationised starch (2.8 meqv/g) as the fixing agent.

Stock starch: Raisamyl 125.

    ______________________________________                                                 Cat.starch                                                                              Stock starch                                                                              Ret.agent                                                                            COD                                     Test point                                                                             %         %           %      mg/l                                    ______________________________________                                        1(0 test)                                                                              0.0       0.0         0.0    3485                                    2        0.0       0.0         0.05   3050                                    3        0.0       0.5         0.05   3050                                    4        0.6       0.5         0.0    2715                                    5        0.3       0.5         0.05   2770                                    6        0.6       0.5         0.05   2625                                    ______________________________________                                    

EXAMPLE 3

Stock: TMP, pH 4.6

Turbidity, COD.

Two batches of cationic starch and two commercial polyDADMACs werecompared.

    ______________________________________                                                     Dosage     Turbidity                                                                              COD                                          Chemical     %          (1:10)   mg/l                                         ______________________________________                                        0 test                  154      2385                                         PolyDADMAC 1 0.2        113      2325                                         Mobil        0.4        68       1795                                                      0.6        44       1595                                         Cationised   0.2        46       1775                                         starch 1     0.4        20       1535                                         (2.6 meqv/g) 0.6        15       1585                                         Cationised   0.2        33       1765                                         starch 2     0.4        20       1595                                         (2.8 meqv/g) 0.6        13       1420                                         PolyDADMAC 2 0.2        71       2020                                         Basf         0.4        32       1610                                                      0.6        19       1430                                         ______________________________________                                    

EXAMPLE 4

Stock: MWC reject, pH 8

Turbidity, charge state, COD.

Two cationic starches and commercial polyDADMAC.

    ______________________________________                                                   Dosage  Turbidity Charge state                                                                           COD                                     Chemical   %       (1:10)    μeqv/l                                                                              mg/l                                    ______________________________________                                        0 test             288       2830     1845                                    Cationised 0.2     31        1620     1665                                    starch 1   0.4     17         460     1550                                    (2.8 meqv/g)                                                                  PolyDADMAC 0.2     33        1200     1632                                    Nalco      0.4     26         380     1510                                    Cationised 0.2     29        1620     1757                                    starch 2   0.4     18         690     1665                                    (2.7 meqv/g)                                                                  ______________________________________                                    

EXAMPLE 5

Stock: Double coated reject, ph 7.5

Turbidity, charge state.

Two cationic starches, each 2.5 meqv/g and a commercial polyDADMAC andpolyaminechloride (PAC).

    ______________________________________                                                    Dosage    Turbidity                                                                              Charge state                                   Chemical    %         (1:10)   μeqv/l                                      ______________________________________                                        0 test                         183                                            Cationised  0.03      86       135                                            starch 1    0.06      58       120                                            (non-purif.)                                                                              0.09      41        92                                            PolyDADMAC  0.03      112      149                                            Nalco       0.06      68       113                                                        0.09      49        78                                            Cationised  0.03      64       119                                            starch 2    0.06      35        96                                            (purified)  0.09      25        87                                            PAC         0.12      161      127                                            Oulu        0.24      103      100                                                        0.48      70        98                                            ______________________________________                                    

EXAMPLE 6

Stock: De-inked stock

Turbidity, COD.

Cationic starch (2.8 meqv/g) and commercial polyDADMAC.

    ______________________________________                                                    Dosage      Turbidity                                                                              COD                                          Chemical    %           (1:10)   mg/l                                         ______________________________________                                        0 test                  383      438                                          PolyDADMAC  0.2         52       390                                          Mobil       0.4         49       355                                                      0.6         85       348                                          Cationised  0.2         40       431                                          starch      0.4         33       402                                                      0.6         30       338                                          ______________________________________                                    

EXAMPLE 7

Stock: test stock mixture (50% groundwood stock+50% coated reject), pH8.4

Turbidity

Two cationised starches and commercial polyDADMAC and polyimine.

    ______________________________________                                                         Dosage  Turbidity                                            Chemical         %       (1:10)                                               ______________________________________                                        0 test                   447                                                  PolyDADMAC       0.15    90                                                   Basf             0.2     63                                                                    0.25    46                                                   Cationised       0.15    95                                                   starch 1         0.2     60                                                   (non-purif.)     0.25    48                                                   (2.7 meqv/g)                                                                  Polyimine        0.15    72                                                   Basf             0.2     49                                                                    0.25    40                                                   Cationised       0.15    65                                                   starch 2         0.2     46                                                   (purified)       0.25    39                                                   (2.6 meqv/g)                                                                  ______________________________________                                    

EXAMPLE 8

Stock: groundwood stock, pH 5.3

Turbidity, COD.

Two cationic starches and a commercial polyDADMAC.

    ______________________________________                                                    Dosage      Turbidity                                                                              COD                                          Chemical    %           (1:10)   mg/l                                         ______________________________________                                        0 test                  267      979                                          Cationised  0.05        132      718                                          starch 1    0.1         53       620                                          (2.6 meqv/g)                                                                              0.15        39       595                                                      0.2         30       589                                          PolyDADMAC  0.05        187      807                                          Nalco       0.1         86       645                                                      0.15        58       602                                                      0.2         47       575                                          Cationised  0.05        168      805                                          starch 2    0.1         68       660                                          (2.1 meqv/g)                                                                              0.15        46       604                                                      0.2         31       581                                          ______________________________________                                    

EXAMPLE 9

Stock: Double coated reject, pH 7.5

Turbidity

Three cationised starches of different cationisation degrees and acommercial polyDADMAC.

    ______________________________________                                                         Dosage  Turbidity                                            Chemical         %       (1:10)                                               ______________________________________                                        Cationised       0.1     40                                                   starch 1         0.3       5.1                                                (2.8 meqv/g)                                                                  Cationised       0.1     71                                                   starch 2         0.3     32                                                   (1.8 meqv/g)                                                                  Cationised       0.1     155                                                  starch 3         0.3     31                                                   (1.0 meqv/g)                                                                  PolyDADMAC       0.1     57                                                   Nalco            0.3       10.4                                               ______________________________________                                    

EXAMPLE 10

Stock: Test stock mixture (50% groundwood stock+50% coated reject ), pH8.4

Turbidity

Three starches of different cationisation degrees.

    ______________________________________                                                         Dosage  Turbidity                                            Chemical         %       (1:10)                                               ______________________________________                                        Cationised       0.1     340                                                  starch 1         0.2     248                                                  (0.14 meqv/g)    0.3     192                                                  Cationised       0.1     270                                                  starch 2         0.2     190                                                  (0.57 meqv/g)    0.3     135                                                  Cationised       0.1     145                                                  starch 3         0.2      30                                                  (2.5 meqv/g)     0.3      19                                                  ______________________________________                                    

Based on the above test results it can be observed that the results ofusing a cationic starch polymer in the reduction of interferencesubstances in water circulation fibre suspensions are of the same orderas those when using polymers traditionally used for the purpose.

I claim:
 1. A method for reducing the amount of anionic colloidalinterference substances in the water circulation in web formationprocesses involving dewatering of wood-based fibre suspensions thatcontain added filling and sizing agents, which comprises addingcationized ammonium starch having a charge density of 1.5-3.5 meqv ofN/g to the fibre suspension in an amount of approximately 0.04-0.5% ofthe fibre dry matter in order to fix said anionic colloidal interferencesubstances from the circulation water to the fibres to be subjected tothe web formation, and wherein said cationized ammonium starch is addedat a stage preceding adding any filling and sizing agent to said fibresuspension.
 2. A method as claimed in claim 1, wherein said cationizedstarch is a starch ether cationized with2,3-epoxy-propyl-trimethylammoniumchloride.
 3. A method as claimed inclaim 2, wherein said cationized starch has a charge density of 2-3 megvof N/g.
 4. A method as claimed in claim 3, wherein said cationizedstarch is added to the fibre suspension before the web formation stage.5. A method as claimed in claim 1, wherein said cationized starch isadded to the fibre suspension before the web formation stage.
 6. Amethod as claimed in claim 2, wherein said cationized starch is added tothe fibre suspension before the web formation stage.